The present invention generally relates to an electrophotographic copying apparatus and more particularly, to a toner replenish control system of the electrophotographic copying apparatus.
Commonly, according to a conventional electrophotographic copying apparatus, a constant developing bias voltage is usually applied to a developing electrode in a developing device so that fogging of copied image may be prevented. However, this type of copying apparatus as described above has such a disadvantage that when an original document to be copied is not of good quality, copied images therefrom may also be poor in quality in some cases, since developing characteristic is kept constant at all times regardless of condition of latent image to be developed. For example, when an image of the original document per se is light in shade, the copied image therefrom tends to be also light in shade, while in the case where non-image portions (i.e. background portions) of the original document are colored, unsightly copied images are apt to be produced, with formation of undesirable fogging at the non-image portions.
In order to avoid such problems as described above, there has conventionally been proposed a self-bias system. According to the system, the developing electrode is grounded via an impedance element such as self-bias resistor, and a developing bias power source, and thus, effective developing bias voltage can be controlled according to an overall density of the original document, i.e. an average potential of the entire electrostatic latent image so that a desired copied image can be obtained. For example, when the entire original document is light in shade, the potential of the electrostatic latent image is lowered, so that a desirable copied image, which is darker in shade than the actual original document, may be obtained. Therefore, according to the self-bias system as described above, an operator can obtain a desired copied image almost without any special operation of the copying apparatus with respect to an ordinary original document, and only when he still desires to obtain dark copied images even by neglecting formation of the fogging, he may control the density of the copied image by adjusting amount of exposure and the like.
In the meanwhile, in the developing device of the copying apparatus as described above since toner contained in a developer is consumed according to the developing of the electrostatic latent image so that toner concentration, i.e. proportion of weight of the toner to the developer, is lowered, fresh toner must be replenished to the developer accommodated in the developing device. In order to accomplish such toner replenishment, conventionally there has been provided a toner replenish control system in which the amount of the toner to be replenished to the developer in the developing device is controlled so as to maintain the toner concentration constant. A typical toner replenish control system conventionaly proposed includes a black reference image provided adjacent to a platform for an original document, and light image of the black reference image is projected onto a surface of a photoreceptor so that a corresponding reference latent image is formed on the surface of the photoreceptor. The reference latent image is subsequently developed by the developing device. The toner replenish control system further comprises a light emitting element for emitting light to the developed reference latent image and a light receiving element for receiving reflection light from the developed reference latent image, the light receiving element producing signal which corresponds to an amount of the reflection light received by the light receiving element, i.e. amount of the toner adhering to an area of the latent image which is proportional to the concentration of toner in the developing material, and toner replenish control device for controlling the amount of fresh toner to be replenished so as to maintain constant the concentration of the toner in the developing material.
However, if the self-bias system and the toner replenish control system would be simultaneously employed for the electrophotographic apparatus, there may arise such a disadvantage that, since a size of the reference latent image, in other words, retained charge at the area of the reference latent image, may vary due to modification of an original document image projecting magnification, the effective developing bias voltage controlled by means of the self-bias resistor also varies with a consequent variation in the amount of toner adhering to the area of the reference latent image. As a result, the toner replenishing can not be correctly controlled, thus, a desired copied image may not be obtained. For example, at a reduced size magnification, the size of the reference latent image is reduced, and this results in that the retained charge at the latent image becomes less, so that an absolute value of the effective developing bias voltage produced by the self-bias resistor becomes less than that at equal size magnification, thus amount of toner at a unit area adhering to the reference latent image becomes larger than that at equal magnification. Therefore, although toner should be replenished, the toner is not actually replenished to the developer, and thus the density of the copied image become less than a desired density.
Such a fact as described above has been proved by experiments carried out by the present inventor in which an apparatus as shown in FIGS. 1(A) and 1(B) was employed.
The apparatus of FIG. 1(A) generally includes a photoreceptor drum 1 applied with a photosensitive layer of CdS.nCdCO.sub.3 resin on the peripheral outer surface thereof and rotatably disposed below and adjacent to an original document platform 28 of a transparent material such as glass or the like, and a developing sleeve 7 provided adjacent to the photosensitive surface layer of said photoreceptor drum 1 in a relation parallel to an axis of said drum 1 as shown. The photoreceptor drum 1 has a width of 310 mm and is charged at -600 V by a corona charger (not shown), while the developing sleeve 7 disposed adjacent to the photoreceptor drum 1 has the same width as that of the photoreceptor drum 1. The developing sleeve 7 is grounded via a self-bias resistor R.sub.0 and a developing bias power source 10. The self-bias resistor R.sub.0 may be connected to either the input terminal of the developing bias power source 10 as shown in FIG. 1(A) or to the output terminal of the developing bias power source 10 as shown in FIG. 1(B). The self-bias resistor R.sub.0 has resistance of 200 M.OMEGA., while the developing bias power source 10 has voltage of -300 V. In the meanwhile, a longitudinal plate 30 with a reference image 29, i.e. reference pattern, is arranged adjacent to the platform 28 for the original document in a position at a starting side of a light scanning for the purpose of reading the original document. The reference image 29 is located at a central portion of the plate 30 which forms a white background, and has a width of 60 mm. The color of the reference image 29 is black with a density at 1.5.
The results of the experiments excuted by using the apparatus as described above are as shown in the following table.
______________________________________ equal size reduced size magnifi- magnifi- cation (.times. 1.0) cation (.times. 0.647) ______________________________________ effective developing bias -345 V -321 V voltage during developing the reference latent image amount of toner adhering 0.6 mg/cm.sup.2 0.657 mg/cm.sup.2 to the area of the reference latent image ______________________________________